This invention relates to a tamperproof closure suitable for use in packaging carbonated beverages.
Due to the economy of manufacture and availability of raw material, the utilization of thermoplastic closures in packaging carbonated beverages is becoming more popular. To be commercially acceptable, the closure must have tamperproof qualities. A highly successful tamperproof system for use on thermoplastic closures is the one disclosed in U.S. Pat. No. 4,206,851. This system utilizes a fracturable band which can be heat shrunk into an interfering fit with a container flange. The fracturable band is carried by a plurality of non-fracturable ribs attached to the lowermost end of the closure sidewall. Attempted removal of the closure from the container results in fracture of the band as it attempts to override the container flange. There are other tamperproof systems, such as the ones shown in U.S. Pat. No. 4,033,472 and British Patent Specification 1,384,370, which also utilizes a tamperproof band which needs to be heated so that it can achieve a position of interfering fit with a container flange.
In designing the total package, either the container and/or the closure must be designed to prevent premature release of the closure from the container. This premature release phenomenon is most often experienced as the user turns the closure to begin its removal from the container. As the closure is turned, it moves axially upward thus breaking the seal between the top of the closure and the top of the container. Upon loss of the seal, pressurized gas from the container enters between the sidewall of the closure and the container tending to bulge the closure sidewall outwardly. As the closure of the sidewall bulges outwardly, the closure threads are pulled away from engagement with the container threads and the connection between the container and closure is tenuous at best. If the gas is under sufficient pressure, the closure will be released from the container since the container-closure thread engagement is insufficient to contain the pressurized gas. This release is oftentimes with great force thereby presenting danger to the user.
To obviate the dangers presented by premature release, it has been suggested that a vent groove be cut on the inside sidewall of the closure. See U.S. Pat. No. 3,888,347. By using the vent groove, the pressurized gas is not trapped between the closure sidewall and the container, but rather is allowed to pass harmlessly to the atmosphere through the vent groove. Combining a tamperproof system which utilizes heat application with a venting system, such as the one described above, can present a difficult problem. The problem lies in the fact that application of heat to the tamperproof band cannot be done very precisely and that, oftentimes, heat intended for the band also reaches the lower closure sidewall. Heating of the sidewall can cause it to shrink inwardly and make intimate contact with the container flange or container neck. When this occurs, the function of the venting system is compromised as the shrunken portion of the sidewall which is in contact with the container interferes with gas escapement.
The degree and incidence of shrinkage is increased when the lower portion of the sidewall is thinned out so that it flares outwardly from the container flange. The flare configuration is desirable since it aids in placement of the cap on the container as it goes through the capping line. The flaring is also desirable as it provides a space between the closure sidewall and the container flange. Of course, by thinning out the lower portion of the closure sidewall, this thinned sidewall portion will more likely reach its heat shrinking temperature if it receives stray heat from the source used to apply heat to the tamperproof band.
Therefore, it is an object of this invention to provide a thermoplastic closure having a heat-shrinkable tamperproof band and a pressurized gas venting system which are compatible with each other.